Well-known golf balls include solid golf balls comprising a solid core of hard rubber and a cover based on an ionomer resin and wound golf balls comprising a wound core consisting of a center and thread rubber wound thereon and a balata cover. Both types of golf balls have a plurality of dimples distributed over the entire spherical surface. In a common procedure, logo and other marks are printed onto the dimpled spherical surface, and a paint, typically clear paint is applied for the purposes of protecting the cover and improving the outer appearance before the ball is completed as a commercial product.
Referring to FIG. 3, the dimple is described in detail. FIG. 3 is a fragmental enlarged view showing the radial cross-sectional shape of one dimple 12 formed in the surface of a golf ball prior to paint coating. Most often, the dimple 12 is circular in plane shape and has a diameter of roughly 3 mm. A plurality of such dimples are distributed at a close spacing over the entire ball surface.
In general, dimples are formed in the golf ball surface as recesses or indentations. The cross-sectional shape of dimples is defined by a portion of a curved surface such as a circle, ellipse or hyperellipse. For example, the cross-sectional shape of the curved surface of the dimple 12 in FIG. 3 is a portion of a circle.
The dimple is circumscribed by an upper edge 26 which is continuously connected to a land 14 of the golf ball surface where no dimples are formed. The edge 26 is generally beveled from the land 14 as a steep slope to form the dimple. The edge 26 is formed angular prior to paint coating and somewhat rounded after paint coating.
As a general rule, the diameter S of the dimple 12 is determined on the basis of the geometry (often circular) circumscribed by the dimple edge 26. The depth or deepest value of the dimple is given by the distance from a plane (circle) 28 circumscribed by the edge 26 to the bottom or deepest point 24, rather than the distance between the bottom 24 and the original ball surface depicted by an imaginary extension line 15 drawn on the assumption that the ball has no dimples. Accordingly, the edge 26 of the dimple becomes a very important basic point when the diameter and depth of the dimple are determined.
However, when paint is applied to dimples having angular edges, the paint coating becomes a largely varying coating which is thin near the edge 26, because of the steep inclination from the edge 26 toward the bottom of the dimple, and forms a buildup at the bottom of the dimple. Besides, when logo and other marks are printed prior to painting, there is a likelihood that print skips occur in areas from the edge to the inside of dimples.
In particular, the paint coating thickness variations in dimples are serious in the sense that even if dimples are formed exactly to the designed configuration prior to painting, the dimples on the ultimately finished golf ball are more or less altered by the paint coating, resulting in a lower dimple precision.
An object of the invention is to provide a dimpled golf ball which is minimized in print skip and coating disparity and can maintain the desired dimple precision even after paint coating.
The invention provides a golf ball having a plurality of dimples formed in its surface. Each dimple prior to paint coating consists of a major concave surface portion defining a recess in the ball surface and a peripheral convex surface portion circumscribing the major concave surface portion. The dimple has a bottom, a peripheral edge and a depth therebetween. The peripheral convex surface portion connects to the major concave surface portion at a point of inflection which is located at a distance of 10 to 50% of the dimple depth from the dimple edge. Preferably the peripheral convex surface portion has a radius of curvature of 0.3 mm to 3.0 mm.
The invention has been made in consideration of the process for the manufacture of a golf ball having a plurality of dimples in its surface, involving the steps of printing logo and other marks and applying a clear paint to the ball surface where dimples of the desired design have been formed. The dimples in the ball surface prior to paint coating are configured to include a major curved surface portion defining a recess in the ball surface and a peripheral curved surface portion connected to the major curved surface portion. The boundary or point of inflection between the major and peripheral curved surface portions is optimized relative to the dimple depth. Then logo and other marks can be printed on the ball surface across the dimples without skipping. When a clear paint is coated thereon, a uniform paint film is obtainable. The painted golf ball maintains the precision of dimples substantially unchanged from prior to painting. It has also been found that paintability is further improved when the peripheral convex surface portion of the dimple has a radius of curvature of 0.3 to 3.0 mm.